Various types of resin or plastic parts have been used in a vehicle such as an automobile. As such a resin part, one is known which is joined by performing vibration welding (see, for example, Patent Document 1).
In the vibration welding, for example, as shown in FIG. 3, a first part 1 is provided with a welding rib 3. A welded surface 2 of the welding rib 3 is attached to a general surface 5 of a second part 4 and vibrated (in a vibration direction 6) so that a vicinity of the welded surface 2 is frictionally melted to weld the two parts (welded portion 7).    [Patent Document 1] Japanese Patent Application Publication No. 2006-218822
However, in a vibration-welded structure obtained by the above vibration welding, as shown in FIG. 4, sufficiently large friction can not be obtained if there is an angle between the welded surface 2 and the vibration direction 6. Accordingly, there are problems in that the welding strength decreases so that failure in welding easily occurs. In particular, it becomes impossible to weld the two parts in a case where the angle between the welded surface 2 and the vibration direction 6 becomes large.